For the following reasons, the present 137Cs source at The Wistar Institute has become inadequate and is limiting the types of research that can be conducted. 1) The small chamber volume makes experiments involving animals larger than mice impossible. 2) A large number of users (82 authorized) and small irradiation volume, which necessitates repeated exposures fo small samples, limit the time that the instrument is available; therefore, many workers are forced to delay or postpone important experiments. 3) The low fixed dose rate of this instrument (154 rads/min) extends user time for experiments requiring high doses, e.g. feeder layers. 4) The antiquated sample transport mechanism limits the minimum dose and, therefore, precludes experiments requiring low doses, e.g., less than 100 rads. The proposals described for usage of the requested cesium source can be categorized as follows. 1) Investigation of the repair of Gamma-ray induced DNA lesions and their relationship to mutagenesis, especially at low doses. These studies will be conducted by isolating Gamma-ray-sensitive mutants and analyzing them for biochemical and mutagenic defects. 2) Whole body irradiation of mice. Large numbers of mice are irradiated for receipt of bone marrow and thymus transplants. Similar experiments with rats are planned should a suitable cesium source become available. 3) Stimulation of thymus-derived (T) cells specific for a wide variety of antigens by irradiated-antigen-presenting cells. These projects are diverse and involve many different immunological models which analyze the T cell responses to viral antigens, histocompatibility antigens, and tumor-specific transplantation antigens. 4) Irradiation of feeder layers required for the growth, activation, and expansion of somatic cell hybrids, cloned tumor cells, and cloned T cells. These proposals comprise a diverse research effort which will benefit from purchase of a cesium source with both large capacity and variable dose rate.